I. Field of the Invention
This invention relates, in general, to electrical control circuits and, more specifically, to resistive controllers which measure the electrical resistance of a substance and, if the measured resistance falls within a certain predetermined range, actuate an external device.
II. Description of the Prior Art
Resistive controllers are known for measuring the electrical resistance of a substance and, when the electrical resistance falls within a predetermined range, for actuating an electrical device. Such resistive controllers have typically been used in automatic sprinkler systems for lawns and farms. In such an application, probes are embedded in the soil to measure the resistance, which is indicative of the moisture content of the soil. When the resistance increases past a predetermined value, the resistive controller activates a relay which connects power to an external electrical device, such as a solenoid-operated valve, to turn on water flow to the soil.
Although the prior art resistive controllers function acceptably in sprinkler control systems, they cannot be adapted for use in other applications requiring the addition of other substances to liquid streams, such as the addition of chlorine to swimming pools or the adjustment of the ph levels of a liquid. The resistive controllers disclosed in the prior art are activated by increasing resistance in the substance being monitored and, therefore, cannot be utilized in those applications which require activation when the measured resistance decreases or falls below a predetermined minimum value. Furthermore, in those applications requiring the addition of a second substance, other than water, to a first substance being monitored, an alarm and immediate shut down of the system is required if the levels of both substances are not balanced within a predetermined range within a selected period of the time. This situation occurs when there are insufficient quantities of second susbstance necessary to balance the constituent levels of the system or a defect has occurred in the application apparatus. None of the resistive controllers utilized in the prior art disclose any means of providing an alarm and immediate shut down of the system if the levels of the substances involved are not within prescribed limits within a selected period of time after the addition of the second substance to the first has been initiated.
Furthermore, the prior art resistive controllers are actuated immediately when the measured resistance reaches a predetermined level. This feature leads to rapid on and off operation as well as the possibility of a false activation as the measured resistance approaches the predetermined resistance level.
Thus, it would be desirable to provide a resistive controller which measures a variable electrical resistance of a substance and, when the measured resistance reaches a predetermined level, which actuates an electrical device to initiate the addition of a second substance to the substance being monitored. It would also be desirable to provide a resistive controller which automatically de-energizes the electrical device if the measured electrical resistance of the substance being monitored is not within prescribed limits within a selected period of time after the addition of the second substance to the first has been initiated. It would also be desirable to provide a resistive controller which is responsive to either increasing and decreasing levels of electrical resistance for actuating an electrical device. Finally, it would be desirable to provide a resistive controller which does not oscillate between on and off states after actuation.